Computations to account for composition of the Mediterranean’s Messinian gypsum and halite

A simple box model is developed to access the astronomical-modulated exchange of Atlantic seawater with the Mediterranean during the Late Miocene salinity crisis. Key to the calculations is an activity coefficient that reduces the rate of evaporation as salinity increases. The model uses a narrow and shallow portal in order to get salinity to increase to saturation for gypsum, halite and the most soluble potash and magnesium salts. Flow through the Atlantic entry portal changes in direction as climate oscillates from arid to wet during each precession cycle. The model addresses the geochemical riddle of “low salinity gypsum” with calculations showing that rain and rivers supply eight times more water to the Mediterranean brine than seawater. The sulfate isotopes in gypsum come in with the ocean and those in the water of hydration in gypsum from atmospheric precipitation. The evolving chloride, sulfate, potassium and magnesium ions observed from fluid inclusions in gypsum and halite are reproduced in the calculations. The mass of computed halite is approximate to the volume of acoustically-transparent halite observed in reflection profiles. The rates of gypsum, halite and kainite precipitation diminish with time as the result of the decreasing activity coefficient and the associated reduction in the amount of Atlantic inflow. The evaporative model’s reduction in the Atlantic-Mediterranean exchange conforms with the sedimentological and geochemical observations of the gypsum deposits on margins and halite on deep basin floors.